<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script src="../jquery-1.11.2.min.js"></script>
</head>
<body>

<form action="test.php">

</form>

<script>
    $(function () {
        $('form').attr('class','aaa');
    })
</script>
</body>
</html>